1. Field of the Invention
This invention relates to a method for recording and erasing information by irradiating a recording medium with a laser beam. More particularly, the present invention is concerned with a method for recording and erasing information by irradiating a recording medium with a single laser beam which is applied to the recording medium with two different intensities, respectively, for recording information on a recording medium and for erasing information recorded on a recording medium. According to the method of the present invention, both the recording and erasing of information can be easily performed using a simple optical system by changing only the laser beam intensity.
2. Discussion of Related Art
Recently, according to the increasing amount of information, the utilization of optical disks and optical cards as materials for recording information has been increased because of the large amount of information that can be recorded on optical disks and optical cards at a high density and the non-destructive readout of the recorded information on the optical disks and the optical cards can easily be made by means of a laser beam.
With respect to the optical disks, the two types known are a write-once optical storage type and a reversible optical storage type.
In the case of the former type of optical disk, information can be recorded only once and the recorded information cannot be erased. On the other hand, in the case of the latter type of optical disk, information can be recorded and erased repeatedly.
With respect to the reversible optical storage type of optical disk, two types of recording materials are known. One is a magneto-optical recording material and the other is a phase-change recording material In the case of the magneto-optical recording material, information is recorded by utilizing occurrence of the difference in direction of magnetization by irradiation with a laser beam, and the recorded information is read out by utilizing the difference in the plane of polarization of a laser beam reflected by the recording material, in which the difference in the plane of polarization is ascribed to the difference in the direction of magnetization. On the other hand, in the case of the phase-change recording material, information is recorded by utilizing occurrence of the difference in the state of a recording material by irradiation with a laser beam, that is, amorphous or crystalline, and the recorded information is read out by utilizing the difference in the optical properties, which difference is ascribed to the difference in the state of a recording material. In the case where a magneto-optical recording material is used for recording and reading out information, it is necessary to use an apparatus comprising a magnetic recording system and an optical reading system. On the other hand, in the case where a phase-change recording material is used for recording and reading out information, both the recording of information and the reading out of the information can be conducted using an optical system only. Accordingly, the phase-change recording material is advantageous in that an apparatus for recording and reading out information is simple as compared with the case where the magneto-optical recording material is used.
In the case of the phase-change recording material, another mode for recording and erasing information is conceivable. That is, information is recorded on the recording material utilizing occurrence of the difference in volume by irradiation with an electron beam between the irradiated area and the non-irradiated area. Utilizing the difference in the volume caused by the irradiation with an electron beam, information is recorded at a high density as compared with the case where the difference in the state of a recording material is utilized. The utilization of the difference in the volume of a recording layer caused by the irradiation with an electron beam has not yet been realized at the present time, but it is expected to come true in the near future.
With respect to the recording layers of the phase-change recording materials, it is known to use various Te alloys, such as a Te-Ge alloy [Appl. Phys. Lett., 49 (9), p. 502, (1986)]and Te-Ge-Sn alloy [Appl. Phys. Lett., 46 (8), p. 734, (1985), and various Se alloys, such as an Sb-Se alloy [Appl. Phys. Lett., 48 (9), p. 1255, (1986)] and an In-Se-Tl alloy [Appl Phys Lett., 50 (11), p. 667, (1987)]. The recording layers of these alloys can be amorphized when the recording layers are melted and then rapidly cooled. The recording layers can be crystallized when the recording layers are melted and then gradually cooled, or when the recording layers are heated for a predetermined period of time at temperatures higher than the critical temperatures for crystallization of the recording layers. The recording layers have different optical properties between the amorphous state and the crystalline state. Therefore, information can be recorded on the recording materials utilizing the occurrence of the difference in optical properties between the amorphous state and the crystalline state. For example, the amorphous state and the crystalline state may be designed so as to correspond to the recorded state and the erased state, respectively. In this case, the recording of information can be effected by the rapid heating and the rapid cooling of a recording layer, and the erasing of the recorded information can be effected by the gradual heating and the gradual cooling of the recording layer. Therefore, by repeating the rapid heating-rapid cooling and the gradual heating-gradual cooling, the recording and erasing can be repeatedly carried out. The crystallization and amorphization of a recording layer can be conducted by irradiating with a laser beam. Of course, alternatively, the crystalline state and the amorphous state may also be designed so as to correspond to the recorded state and the erased state, respectively. For easy understanding, however, an explanation will be made hereinbelow with respect to the case where the amorphous state and the crystalline state correspond to the recorded state and the erased state, respectively, unless otherwise described.
As mentioned above, when information recorded on the above-mentioned conventional recording material is erased, that is, the crystallization of the recording layer is effected, gradual heating and gradual cooling must be conducted, whereas when information is recorded on the conventional recording material, that is, the amorphization of the recording material is effected, rapid heating and rapid cooling must be conducted. The rapid heating and the rapid cooling can be performed by a short irradiation period with a laser beam of a high intensity, whereas the gradual heating and the gradual cooling can be performed by a long irradiation period with a laser beam of a low intensity. In practice, however, performing of both the short and long irradiation periods are accompanied by difficulties. In operating the phase-change type information recording and erasing system, a recording layer is migrated relative to a laser beam at a constant speed. When a laser beam spot is applied to a recording layer for recording or erasing information at a certain point on the recording layer, the point continues to be irradiated with the laser beam during the passage of the point through the width of the laser beam spot. Therefore, whether the irradiation period of time is short or long, would depend upon the width of the laser beam spot. Therefore, for performing both the short irradiation period for amorphization of a recording material and the long irradiation period for crystallization of a recording material, two laser beams have conventionally been used, i.e. a laser beam having a circular irradiation spot and a laser beam having an elliptic irradiation spot. Such an irradiation system using two laser beams has disadvantages in that it is necessary to use a complicated apparatus which comprises two laser beam irradiation systems and that the two laser beam irradiation systems must be so arranged that the axis of one of the laser beams is coincident with the axis of the other laser beam with high precision in order to repeatedly record and erase information with accuracy. However, it is difficult as a practical matter, to coincide the axes of both laser beams with each other. Particularly in the case where high speed recording and erasing is intended to perform, it is necessary to use a laser beam for the crystallization of a recording medium, which has an ellipitic beam spot of a considerably large length in its long axis. However, the use of such laser beam is not practical.